Portable data processing apparatus, such as workstations and other devices, may be connected to a local area network (LAN) via wireless links, which may use infrared or radio signals as a communications medium. The connection of the portable data processor to the LAN is through a base station. The base station is an intelligent device that may appear to the LAN as a bridge, a gateway or a concentrator. There may be multiple mobile units connected by wireless links to a single base station, the base station providing a protocol that enables the mobile units to communicate with the LAN. The mobile units may appear to the LAN to be a subnet or to be part of the same logical LAN. Whatever protocol is employed, a mapping exists for ensuring that network data messages, or datagrams, that are addressed to a mobile unit are routed to the appropriate base station for transmission to the addressed mobile unit. The mapping also ensures that datagrams originating with a particular one of the mobile units are correctly routed into the LAN.
A natural feature of a network of portable wireless devices is that a given portable device will continually visit a series of base stations. One example of a useful application is in a hospital environment wherein medical personnel are in contact with a patient database through a wireless network, which in turn is coupled to a wired LAN through which the database is accessible. Base stations may be ceiling mounted in each patient room for sending and receiving information to a handheld data processing device carried by the medical personnel. As the medical personnel make their rounds they can be expected to visit a large number of base stations.
This expected pattern of use gives rise to two distinct sets of problems. Firstly, a mechanism is required to inform senders of which base station is currently hosting a moving data processor. Under one known network protocol, TCP/IP by example, an Address Resolution Protocol (ARP) can perform this function.
A second problem relates to providing a mechanism to conceal from a remote sender that the moving workstation has intermittently disconnected from the network. For some protocols, for example those based on NETBIOS sessions, interruption of the session leads to its termination. For TCP/IP, failure to deliver a frame of data results in some number of retries followed by a timeout and disconnection from the network.
In that movement or migration of the mobile data processors between base stations may be a frequent and on-going activity for some wireless network applications it can be appreciated that it is important to provide mechanisms to sustain the illusion of a permanent, if virtual, connection of the workstation to the network. To provide such a mechanism it is important to also provide an efficient and reliable method for a given base station to "hand-off" a moving data processor to another base station without loss of connection from the network and without a loss of data being directed from the network to the mobile data processor.
In U.S. Pat. No. 4,456,793, issued Jun. 26, 1984, W. E. Baker et al. describe a cordless telephone system having infrared wireless links between handsets and transponders. The transponders are wired to subsystem controllers which are in turn wired to a system controller. The central controller polls the cordless stations every 100 milliseconds to detect cordless station locations and to identify "missing" cordless stations.
In U.S. Pat. No. 4,807,222, issued Feb. 21, 1989 N. Amitay describes a LAN wherein users communicate with RF or IR signals with an assigned Regional Bus Interface Unit (RBIU). Protocols such as CSMA/CD and slotted ALOHA are employed in communicating with the RBIUs.
In commonly assigned U.S. Pat. No. 4,809,257, issued Feb. 28, 1989 D. Gantenbein, F. Gfeller and E. Mumprecht describe an infrared communication system are assigned to one of a plurality of hierarchical categories.
In commonly assigned U.S. Pat. No. 4,402,090, issued Aug. 30, 1983, F Gfeller et al. describe an infrared communication system that operates between a plurality of satellite stations and a plurality of terminal stations. A host computer communicates with the terminal stations via a cluster controller and the satellite stations, which may be ceiling mounted. Communication with the terminal stations is not interrupted even during movement of the terminal stations.
In Japanese Publication 89-169063/23 there is described a voice system that stores a recording of a voice message when a mobile station cannot be reached. Subsequently, the mobile station transmits a message recovery code in response to which a base station sends the recorded message.
In International Application WO 88/07794, published Oct. 6, 1988, G. Vacon et al. disclose apparatus and method for coupling two LANs by means of a wireless radiation link.
In IBM Technical Disclosure Bulletin, Vol. 20, No. 7, Dec. 1977 F. Closs et al. describe the use of both line-of-sight and diffuse transmission of infrared signals for wireless communications between a ceiling-based controller and a plurality of terminals.
In IBM Technical Disclosure Bulletin, Vol. 24, No. 8, page 4043, Jan. 1982 F. Gfeller describes general control principles of an infrared wireless network incorporating multiple ceiling mounted transponders that couple a host/controller to multiple terminal stations. Access to the uplink channel is controlled by a Carrier Sense Multiple Access/Collision Detection (CSMA/CD) method.
It is thus an object of the invention to provide method and apparatus for providing a continual connection of a migrating data processor to a network employing a wireless link.